1. Field of the Invention
The present invention relates to a galvannealed steel sheet having excellent sliding performance during press-forming and to a method for manufacturing thereof.
2. Description of Related Arts
Galvannealed steel sheets are used in wide industrial fields centering on the automobile body owing to the excellent weldability and paintability compared with those of galvanized steel sheets. The galvannealed steel sheets are, however, difficult in smooth entering into a die during press-forming at a portion that is sandwiched between the die and a bead, where the sliding resistance increases. In other words, the galvannealed steel sheets do not have superior sliding performance and likely induce fracture compared with ordinary cold-rolled steel sheets.
In a galvannealed steel sheet, heat treatment applied after zinc-coated makes iron in the steel sheet and zinc in the coating layer diffuse to form an Fexe2x80x94Zn alloy phase. Generally, the alloy phase consists of xcex93 phase, xcex41 phase, and xcex6 phase, gives Fe concentration decreasing in the order of xcex6 phase, xcex41 phase, and 70  phase, and has a tendency of decreasing the hardness and the melting point in that order. Accordingly, from the point of sliding performance during press-forming, it is effective to form an alloy phase containing large amount of Fe, having high hardness and high melting point, and therefore being difficult to induce adhesion.
If, however, an alloy phase containing large amount of Fe is formed, a hard and brittle xcex93 phase likely appears on the interface between the coating layer and the steel sheet, which likely induces what is called the powdering, or a phenomenon of separation of coating layer from the interface during press-forming.
As a means to provide both the sliding performance and the anti-powdering property, JP-A-1-319661, (the term xe2x80x9cJP-Axe2x80x9d referred herein signifies the xe2x80x9cunexamined Japanese patent publicationxe2x80x9d), discloses a method of forming a hard iron-base alloy layer as a second layer on the coating layer using electrodeposition coating treatment or the like.
The method, however, requires additional coating treatment after hot dip galvanization, which makes the process complex and significantly increases the cost.
A widely used method for improving the press-formability of zinc-base coated steel sheets is to apply high viscosity lubricant oil on the steel sheet. The method, however, raises a problem of generation of coating defects during painting caused by insufficient degreasing, and a problem of instable press-formability caused by lack of oil during press-forming.
To this point, JP-A-53-60332 and JP-A-2-190483 provide methods to improve the press-formability and the weldability by forming an oxide film consisting mainly of ZnO on the surface of coating layer on the zinc-base coated steel sheet using electrodeposition coating treatment, immersion treatment, application and oxidation treatment, heating treatment, or the like.
JP-A-4-88196 provides a method to improve the press-formability and the chemical conversion treatment performance by forming an oxide film consisting mainly of P-oxide on the surface of the coating layer by immersing a zinc-base coated steel sheet in an aqueous solution containing 5 to 60 g/l of sodium phosphate, at 2 to 6 of pH, by conducting electrodeposition coating treatment in the aqueous solution, or by applying the aqueous solution onto the surface of the steel sheet.
Furthermore, JP-A-3-191093 provides a method to improve the press-formability and the chemical conversion treatment performance by forming a Ni-oxide film on the surface of the coating layer on the zinc-base coated steel sheet using electrodeposition coating treatment, immersion treatment, application treatment, application and oxidation treatment, heating treatment, or the like.
Nevertheless, the experiments of forming those types of oxide film on the surface of galvannealed steel sheet, conducted by the inventors of the present invention, not necessarily gave excellent sliding performance.
An object of the present invention is to provide a galvannealed steel sheet that does not generate powdering during press-forming and that assures stable and excellent sliding performance, and to provide a method for manufacturing thereof.
The object is attained by a galvannealed steel sheet having an oxide layer having 10 nm or larger thickness on plateau of coating layer flattened by temper rolling.
The steel sheet can be manufactured by a method for manufacturing a galvannealed steel sheet comprising the steps of: applying hot dip galvanization to a steel sheet; heating the hot dip coated steel sheet to alloy the coating layer; applying temper rolling to the galvannealed steel sheet; and forming a zinc-base oxide layer on the surface of coating layer of the galvannealed steel sheet after the temper rolling.